1. Field of the Invention
This invention relates generally to analog to digital converters and particularly to converters for converting the output of a condition sensing capacitor arrangement to a digital word. More particularly, this invention relates to a converter of the type described which has increased precision and reduced hardware as compared to devices now known in the art.
2. Description of the Prior Art
Air data computers used for aircraft navigational purposes employ capacitor arrangements whereby capacitance changes as a function of atmospheric pressure and is thus a measure of the altitude of the craft. The capacitors provide analog signals which, for most modern day applicatins, must be converted to digital words. Conversion apparatus presently employed involves precision bridge circuits which provide an a.c. voltage related to the capacitance change being measured. The a.c. voltage is then converted to a digital word through the use of precision conversion apparatus such as described in commonly assigned U.S. Pat. No. 3,882,488, issued on May 6, 1975 to Henry R. Kosakowski and Douglas J. Washburn. The conversion system described in the aforenoted U.S. Pat. No. may be used to provide measurements of static and dynamic pressure which are utilized by the air data computer for the aforenoted aircraft navigational purposes.
The bridge circuits involved in the prior art apparatus require a large number of precision passive and active components to obtain absolute voltages related to static and dynamic pressure as is required. Further, a considerable amount of filtering is necessary to reduce the errors associated with the harmonic distortion and power supply variations. This, in turn, imparts frequency sensitivity to the output of oscillators used in the system, thus requiring very stable oscillators. It will be recognized by those skilled in the art that the aforenoted factors represent serious disadvantages in a conversion system for the purposes described.